I. Field of the Invention
The current invention relates to wireless telecommunications. More particularly, the present invention relates to a novel and improved method of compensating for phase and amplitude distortion of multiple signals transmitted through a single channel.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communication system techniques, such as time division multiple access (TDMA), frequency division multiple access (FDMA) and AM modulation schemes such as amplitude companded single sideband (ACSSB) are known in the art. Techniques for distinguishing different concurrently-transmitted signals in multiple access communication systems are also known as channelization. The spread spectrum modulation technique of CDMA has significant advantages over other multiple access techniques.
The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERSxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, and in U.S. Pat. No. 5,751,761, entitled xe2x80x9cSYSTEM AND METHOD FOR ORTHOGONAL SPREAD SPECTRUM SEQUENCE GENERATION IN VARIABLE DATA RATE SYSTEMSxe2x80x9d, both assigned to the assignee of the present invention and incorporated by reference herein. Code division multiple access communications systems have been standardized in the United States in Telecommunications Industry Association TIA/EIA/IS-95-A, entitled xe2x80x9cMOBILE STATION-BASE STATION COMPATIBILITY STANDARD FOR DUAL-MODE WIDEBAND SPREAD SPECTRUM CELLULAR SYSTEMxe2x80x9d, hereafter referred to as IS-95 and incorporated by reference herein.
The International Telecommunications Union recently requested the submission of proposed methods for providing high rate data and high-quality speech services over wireless communication channels. A first of these proposals was issued by the Telecommunications Industry Association, entitled xe2x80x9cThe cdma2000 ITU-R RTT Candidate Submissionxe2x80x9d, hereafter referred to as cdma2000 and incorporated by reference herein. A second of these proposals was issued by the European Telecommunications Standards Institute (ETSI), entitled xe2x80x9cThe ETSI UMTS Terrestrial Radio Access (UTRA) ITU-R RTT Candidate Submissionxe2x80x9d. And a third proposal was submitted by U.S. TG 8/1 entitled xe2x80x9cThe UWC-136 Candidate Submissionxe2x80x9d (referred to herein as EDGE). The contents of these submissions is public record and is well known in the art.
In the CDMA demodulator structure used in some IS-95 systems, the pseudonoise (PN) chip interval defines the minimum separation two paths must have in order to be combined. Before the distinct paths can be demodulated, the relative arrival times (or offsets) of the paths in the received signal must first be determined. The demodulator performs this function by xe2x80x9csearchingxe2x80x9d through a sequence of offsets and measuring the energy received at each offset. If the energy associated with a potential offset exceeds a certain threshold, a demodulation element, or xe2x80x9cfingerxe2x80x9d may be assigned to that offset. The signal present at that path offset can then be summed with the contributions of other fingers at their respective offsets. The use of CDMA searchers is disclosed in U.S. Pat. No. 5,764,687, entitled xe2x80x9cMOBILE DEMODULATOR ARCHITECTURE FOR A SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEMxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein.
In the CDMA receiver structure used in some IS-95 systems, data passing from transmitter to receiver is divided into frames which are transmitted at fixed time intervals. Depending on the varying amount of data to be transmitted during each interval, the transmitter places the data into one of several sizes of frame. Since each of these frame sizes corresponds to a different data rate, the frames are often referred to variable-rate frames. The receiver in such a system must determine the rate of each received frame to properly interpret the data carried within the received frame. Such rate determination methods often include the generation of frame quality metrics, which may be used to assess the level of uncertainty associated with the determined frame rate. Methods of performing rate determination and generating frame quality metrics are disclosed in U.S. Pat. No. 5,751,725, entitled xe2x80x9cMETHOD AND APPARATUS FOR DETERMINING THE RATE OF RECEIVED DATA IN A VARIABLE RATE COMMUNICATION SYSTEMxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein.
Signals in a CDMA system may be complex PN spread as described in U.S. patent application Ser. No. 08/856,428, entitled xe2x80x9cREDUCED PEAK TO AVERAGE TRANSMIT POWER HIGH DATA RATE IN A CDMA WIRELESS COMMUNICATION SYSTEM,xe2x80x9d filed Apr. 9, 1996, assigned to the assignee of the present invention and incorporated by reference herein, and in accordance with the following equations:
I=Ixe2x80x2PNI+Qxe2x80x2PNQxe2x80x83xe2x80x83(1) 
Q=Ixe2x80x2PNQxe2x88x92Qxe2x80x2PNI.xe2x80x83xe2x80x83(2) 
where PNI and PNQ are distinct PN spreading codes and Ixe2x80x2 and Qxe2x80x2 are two channels being spread at the transmitter.
As described in cdma2000, transmission signals are constructed utilizing orthogonal Walsh coding, with one Walsh code used to transmit a pilot sub-channel signal. The orthogonal Walsh sub-channels used to construct such transmission signals are added together before being transmitted, and travel through the same transmission channels or pathways before being received at the receiver. Each transmission channel, by its inherent nature, alters the phase and amplitude of the signals passing through it, and also adds a component of thermal noise. These channel characteristics change with any movement by transmitter or receiver, but may vary over time even when both receiver and transmitter are stationary. Channel characteristics generally change very slowly compared with the data symbols transmitted through the channel.
Some CDMA receivers employ circuits which estimate the phase and amplitude distortion of the channel. These estimates are then used to compensate for channel distortion, enabling more accurate decoding and demodulation of the received signals. One such circuit for estimating phase and amplitude of a channel, and performing a dot product of that output with the demodulated data signal, is described in detail in U.S. Pat. No. 5,506,865, entitled xe2x80x9cPILOT CARRIER DOT PRODUCT CIRCUITxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein. In that described implementation, an all-zero pilot channel is received and used to estimate the channel characteristics. The resultant channel estimates are then used to convert demodulated signals to scalar digital values.
All CDMA signals transmitted on orthogonal sub-channels cause mutual interference to each other, as well as acting as jammers for adjacent cell areas. To enable coherent demodulation of orthogonal sub-channel signals, one sub-channel is often dedicated as a pilot carrier. As detailed in aforementioned U.S. Pat. No. 5,506,865, the pilot carrier is used in the receiver to produce estimates of the channel characteristics. The accuracy of these channel estimates is dependent on the strength of the pilot channel signal. Unfortunately, the pilot channel carries no data, so it is desirable to minimize the pilot transmit power. Conventionally the pilot power relative to the data signal power is selected by balancing between these two factors such that the best overall system performance can be achieved. For this reason, a method of producing accurate channel estimates which does not require increased pilot signal strength is highly desirable.
The present invention describes a method and apparatus for improving the performance of a receiver that receives multiple sub-channel signals transmitted together through a common propagation path, also called a transmission channel. In order to compensate for phase and amplitude distortion introduced into the signals by the transmission channel, the receiver uses a pilot sub-channel signal to estimate the phase and amplitude distortion of the transmission channel. The process of estimating of distortion inherent in the transmission channel is called channel estimation, which is used to produce channel estimates. The invention includes a novel method of utilizing data carrying sub-channels (not the pilot sub-channel) to improve the accuracy of channel estimates. The present invention is applicable to any communication system employing simultaneous transmission of multiple sub-channels and coherent demodulation.
The sub-channel signals within an information signal may be either time division multiplexed (TDMed) or code division multiplexed (CDMed). The exemplary embodiment describes the present invention in the context of the reverse link proposed in cdma2000. Because of overriding commonalties in channel structure, the present invention is equally applicable to reception of the reverse link transmissions according to the candidate submission proposed by the European Telecommunications Standards Institute (ETSI), entitled xe2x80x9cThe ETSI UMTS Terrestrial Radio Access (UTRA) ITU-R RTT Candidate Submissionxe2x80x9d (hereafter WCDMA). Moreover, the present invention is equally applicable to reception of the forward link of these systems.
In cdma2000, the data-bearing sub-channels include a high data rate (e.g. 76.8 kbps) supplemental channel and a low data rate (e.g. 9.6 kbps) fundamental channel. The nominal power of the pilot channel is optimized for demodulation of the fundamental channel (e.g., xc2xc of the fundamental channel power). In order to enable proper demodulation of the high data rate supplemental channel, the cdma2000 standard proposes to increase the pilot power beyond nominal levels when the supplemental channel is in use. In addition, the cdma2000 standard proposed to use different levels of pilot power depending on which of several available data rates the supplemental channel is using.
Varying the pilot power according to data rate causes other difficulties in system design. For example, it requires the receiver to know the data rate in advance in order for the power control loop to behave correctly. This also makes the selection of searching/finger locking more difficult. Moreover, it is desirable to reduce the pilot overhead to improve overall system performance if it can be done without sacrificing demodulation performance.
By enabling the formation of channel estimates based on the fundamental channel signal, the present invention enables a system to achieve superior supplementary channel demodulation performance. If enough channel estimate information can be extracted from the fundamental channel, acceptable supplementary channel demodulation performance may be achieved without varying the pilot power at all. Because the fundamental signal can be transmitted with as much as 4 times the power of the pilot signal, a channel estimate formed using both signals is much more accurate than an estimate based on the pilot signal alone. Subsequent demodulation using the more accurate channel estimate will have improved performance as well.
In cdma2000, the transmit power of the fundamental channel is four times that of the nominal pilot. The combined power of the pilot and fundamental channels would be five times the power of just the nominal pilot channel. A combined channel estimate derived from both the nominal pilot and fundamental channels would be accurate enough for demodulating a cdma2000 supplemental channel. Though increasing the pilot power whenever the supplemental channel is in use would still be an option, it may not be necessary given the enhanced accuracy of the combined channel estimate.
The added accuracy of a channel estimate extracted from the received fundamental channel depends on the use of a correct reference signal, which is optimally identical to the transmitted fundamental channel signal. Any inaccuracy in the decoded symbols used in forming fundamental channel estimates will degrade the quality of the combined channel estimate. Though the supplemental channel is likely to be a packet data channel, which has a high tolerance for frame errors, it may still be desirable to minimize the frame error rate when demodulating the supplemental channel.
In the preferred embodiment of the invention, the received fundamental channel signal is first deinterleaved and forward error correction (FEC) decoded to take advantage of the transmitter""s complementary FEC encoding and interleaving functions. Then, the corrected symbol stream is re-encoded and re-interleaved to produce an ideal replica of the transmitted signal for use as a reference signal by the channel estimator.
In an alternative embodiment of the invention, fundamental channel power is increased as necessary to reduce the fundamental channel error rate. Because decreasing the fundamental channel error rate produces a more accurate channel estimate, increasing fundamental channel power also results in a reduced error rate when demodulating the supplemental channel. When the data rate ratio between the supplemental and the fundamental channels is large, a slight increase in fundamental channel power has little effect on the total transmitted power and hence causes little degradation.
In a more general sense, the present invention can be used where a single channel of information is transmitted. In an alternate embodiment using a single data channel, the channel is artificially split into two physical channels, which are transmitted synchronously at different data rates. Upon receipt, the low rate channel is first demodulated and decoded using pilot based channel estimates. The decoded bits are then re-encoded and used to improve the channel estimates used to coherently demodulate the high data rate supplemental channel. This scheme may enable data throughput which draws nearer to the theoretical capacity limit in a fading environment.